The present invention generally relates to a plasma emission source and, in particular, relates to a source wherein the power transfer efficiency is continuously and automatically maximized.
Plasma emission sources are used to atomize and excite a sample to cause the emission of light at wavelengths which are characteristic of the atomic structure of the sample. The emitted light is detected and measured by a spectrophotometer to complete the analytical process.
In conventional plasma emission sources, radio-frequency (RF) energy is inductively coupled from an RF generator to a plasma torch. Liquid samples are mixed with a solvent, nebulized and delivered into the flame of the torch. Usually, the torch is an argon plasma discharge and the sample plus solvent is carried thereinto by a stream of argon.
As with any RF apparatus, the efficiency of the energy transferred from the RF generator to the load (i.e. the torch) is dependent on the impedance matching therebetween. Hence, modern plasma emission sources include an impedance matching network between the RF generator and the plasma torch.
As it happens, as well known, the impedance of the torch, specifically a loading coil, depends upon both the static and dynamic operating parameters of the plasma emission source. Some of the parameters affecting the impedance of the torch include: changes in the sample and/or solvent; the desired operating temperature of the torch and the efficiency of the nebulizer. To date such changes required the operator to manually fine tune the impedance matching network. In addition, the nebulizer flow adjustments were quite critical in order to help minimize the required manual tuning. Nevertheless, it is quite difficult to maintain the continuous maximum power transfer since these changes are usually dynamic and occur during the actual measuring time.
As a consequence, plasma emission sources presently require excessive RF power input levels to compensate for the relatively poor power transfer to the torch and require frequent readjustment, particularly when solvents are changed.